Natural mortality ogives for the Indian Ocean bigeye and yellowfin tuna stock assessments

Natural mortality for bigeye tuna was estimated based on the maximum ages both in the Indian Ocean and in all oceans, using the method of Hamel and Cope (2022). Sensitivity to updated natural mortality (M) estimates was explored for the 2019 bigeye stock assessment. Sensitivity was similarly explored for the 2021 yellowfin stock assessment, based on M estimates from Hoyle et al (2023). The updated ogives for bigeye tuna tended to have higher M than the 2019 assessment for older fish, and consequently tended to increase the estimated productivity of the stock. In contrast, the updated yellowfin tuna ogives tended to have lower M than the 2021 yellowfin assessment for older fish, which implied lower stock productivity.

Further work is recommended in the following areas: 1) collect further data across all areas of the Indian Ocean for studies of age and growth, with a particular focus on areas with lower historical fishing mortality; and 2) conduct simulations focused on the Indian Ocean stock to explore the potential of stock assessments and auxiliary analyses to estimate M and other model parameters using data from tagging studies.